The present invention relates to an anti-lock control method for preventing locking of automotive wheels of a running vehicle, and, more particularly, to a method and apparatus for estimating a coefficient of friction between a road surface and wheels that are anti-lock controlled.
Generally, in an anti-lock control apparatus for automotive vehicles, to maintain steering ability and running stability during braking, a control unit including a micro-computer which controls brake hydraulic pressure. This brake pressure control also reduces braking distance.
During operation of the anti-lock control apparatus, a control mode for brake hydraulic pressure is determined based on an electric signal indicative of a wheel speed. This wheel speed is detected by a wheel speed sensor. According to the control mode, a hold valve, which is a normally opened solenoid valve, and a decay valve, which is a normally closed solenoid valve, are selectively opened and closed to increase, hold constant and decrease the brake hydraulic pressure.
In such anti-lock control, in many cases the control is performed by the steps of setting a target slip rate for braking, detecting a difference between an actual wheel speed (hereinafter referred as the wheel speed) and a target speed which is a wheel speed calculated from the target slip rate, and determining the required increase or decrease of the brake hydraulic pressure based on this detected difference and acceleration/deceleration of the wheel speed. Therefore, the wheel speed is rapidly converged to the target speed.
However, since the wheel speed varies due to slight changes on a road surface, the above-mentioned anti-lock control suffers from problems such that the decrease and increase of the brake hydraulic pressure is undesirably repeated many times due to the effect of the change on the road surface. As a result, much time is required to converge the wheel speed to the target speed. Also, the period in which a braking force is insufficient is extended because the brake hydraulic pressure is not rapidly and suitably increased. Thus, the braking distance increases and the deceleration of the vehicle is insufficient.